This invention relates generally to materials useful for protecting submerged or submersible structures against marine fouling and to marine identification markers made therefrom.
The classic approach to the prevention or retardation of the growth of marine organisms on submerged or submersible surfaces has been to clad the surfaces with sheet copper or copper alloy. A number of problems and disadvantages arise. Firstly, sheet copper or copper alloy is expensive. Secondly, it can be very difficult to clad an existing structure with copper sheet, because the copper must be kept out of contact with other metallic materials such as steel to avoid galvanic action which not only promotes corrosion but also renders the copper ineffective as an antifouling material. Certainly, it can be difficult physically to attach the copper firmly enough to the surface, e.g. the hull of a ship, for it to remain securely in place over a period of years. There are also other problems, and as a result although sheet copper is an excellent antifouling material, it is not in practice used very widely as a cladding.
With a view to avoiding some of the difficulties associated with sheet copper cladding, antifouling paints have been developed which contain copper oxide. One of the main drawbacks with such paints, however, is that they have a relatively short life, e.g. six months to two years at most.
Another antifouling coating approach is disclosed in U.S. Pat. No. 4,323,599 using copper flakes mixed with an uncured water-insoluble polymer and applying the mixture to the surface to be protected, whereafter the coating is cured. A sufficient amount of copper flake is utilized so the entire thickness of the coating (gel coat) is electrically conductive.
In these known approaches, it has been considered essential to cover the whole of the surface to be protected against fouling, with either the sheet copper (or alloy) or with the copper-containing paint or gel. However, in U.S. Pat. No. 4,375,199, commonly owned herewith, another approach to the problem is described. In particular, described therein is a manner in which antifouling can be achieved by providing a surface which is a continuum of inert material, such as a plastic material, with a multiplicity of tiny, regularly arranged, areas of exposed copper (or copper alloy) therein. Such a surface can be provided by embedding, in a carrier material, a woven or knitted wire mesh, or an expanded metal grid. In the case of the wire mesh, i.e. wire cloth, the knuckles on one side of the wire cloth are exposed at the surface, forming a regular array of tiny copper areas in the surface continuum of carrier material. The opposite face of the wire cloth is embedded in the carrier material and not exposed. This antifouling composite material can be formed in situ on a structure, e.g. the hull of a ship, or can be pre-fabricated in panels for subsequent application to a submerged or submersible structure. The panels can be made in virtually any shape suitable for their intended use. Reference should be made to the aforementioned U.S. Pat. No. 4,375,199 for further details.
It has now been found that antifouling surfaces comprising a continuum of inert carrier material with interspersed small areas of exposed copper or copper alloy can with advantage, be made in a form and in ways other than those described in U.S. Pat. No. 4,375,199, provided certain criteria are met, and can be used in the special application of long life antifouling marker devices.
Although the antifouling structure according to U.S. Pat. No. 4,375,199 can have its mesh embedded in a flexible plastics material, even when such a material is used the final structure is relatively inflexible. This is, of course, due to the inherent inflexibility of the woven copper mesh itself.
For purposes of cladding sub-sea pipes with an antifouling coating, the coating material must either be preformed of an exact shaped fit to the curvature of the pipe or it must be flexible so that it can be shaped onto the pipe surface. However, it is not sufficient merely to be able to bend the material around the pipe because at least at junctions with other pipes and on bends, the antifouling material must also be capable of flexing longitudinally, i.e. stretching. Thus, although the structure of the aforementioned commonly owned patent having its woven copper mesh embedded in a plastic carrier material could flex about a single radius of curvature, it could not stretch.
Another problem in the antifouling field is that in recent years, particularly in connection with off-shore oil and gas rigs and platforms, and associated equipment such as sub-sea pipelines, a need has arisen to be able to identify components, particularly at depth. There are several problems, peculiar to marine environment, which make the provision of satisfactory identification markers difficult. These include the corrosive effects of sea water, the problem of providing a marker which is clearly visible under the limited illumination available, and of course the problem of fouling the marker by marine vegetable and animal organisms.